A diesel engine is equipped with an exhaust gas treatment device including a diesel oxidation catalyst (DOC) disposed in an exhaust passage and a diesel particulate filter (DPF) disposed on the downstream side of the DOC. A diesel particulate filter (DPF) is a device for collecting particulate matters (PM) contained in exhaust gas discharged from the diesel engine. The DPF is normally formed of ceramic or the like in a honeycomb-shaped monolith with adjacent vents closed alternately on the inlet side and the outlet side so that exhaust gas passes through filtering walls which remove PM. The DPF may support a catalyst. Accumulation of PM in the DPF eventually brings about blockage, which leads to not only a decrease in the PM-collecting performance of the DPF but also to an increase in an exhaust pressure and thus has a negative effect on the fuel efficiency. In view of this, it is necessary to perform forced regeneration to remove the PM accumulated in the DPF every time a PM-accumulation amount reaches a predetermined amount or an engine operation time reaches a predetermined time.
Forced regeneration of the DPF is performed by forcedly increasing the inlet temperature of the DPF. In general, the inlet temperature of the DPF is forcedly increased by supplying non-combusted fuel to the exhaust-gas treatment device by post injection, which is to inject fuel after a main-combustion injection timing, and oxidizing the non-combusted fuel with a diesel oxidization catalyst (DOC) so that the temperature of the non-combusted fuel increases. Further, combustion may be supplied by exhaust-pipe injection to an exhaust passage on the downstream side of the engine. The DOC is normally formed of ceramic or the like in a honeycomb-shaped monolith, similarly to the above described DPF, and supports oxidization catalyst on the inner surface of the DOC.
In such an exhaust-gas post treatment device, if a state with a low operation load and a low exhaust-gas temperature continues, SOF or soot of non-combusted fuel or the like adheres to an upstream end surface of the DOC, and blockage of the DOC gradually progresses. If the DOC becomes blocked, the exhaust pressure increases and the fuel efficiency deteriorates, and the non-combusted fuel is not sufficiently oxidized by the DOC and slips, upon forced regeneration of DPF. In response, more and more non-combusted fuel is supplied to the DOC to increase the inlet temperature of the DPF to a predetermined temperature, thereby deteriorating the fuel efficiency. Further, the slipped fuel is oxidized and heated by the DPF supporting a catalyst, and thus promotes abnormal combustion of the PM, thereby raising the risk of heat damage to the DPF. Even further, in a case where the non-combusted fuel upon forced regeneration is performed by the post injection amount, the risk of oil dilution increases.
For instance, Patent Documents 1 and 2 disclose a technique for preventing blockage of DOC. Patent Document 1 discloses a DPF regeneration control device whereby it is possible to prevent blockage of a DOC more efficiently than the conventional technique, and to recover the DOC reliably from the blockage even if the DOC is actually subject to blockage. Specifically, if a blockage parameter related to blockage of DOC detected during automatic regeneration (forced regeneration) of DPF by the first temperature increase unit and the second temperature increase unit exceeds a predetermined blockage threshold for a predetermined period, it is determined that DOC is blocked, and manual regeneration is performed, whose DPF regeneration temperature is higher than that in automatic regeneration. Further, if it is determined from the blockage parameter that DOC is not blocked but in an initial stage of blockage, only the first temperature increase unit continues operation for a predetermined period of time after completion of automatic regeneration, and thereby the temperature of heated DOC is maintained. On the other hand, during normal operation in which forced regeneration (automatic regeneration, manual regeneration) of DPF is not performed, if it is estimated that DOC has been likely to cause blockage, it is determined that DPC is in a blockage risk state, and recovery operation is performed, in which only the first temperature increase unit is operated for a predetermined period of time.
Further, Patent Document 2 discloses combusting and removing non-combusted fuel adhering to the surface of the DOC by maintaining an exhaust-gas temperature at a predetermined temperature with an exhaust-gas temperature maintaining unit after completion of the regeneration process of the DPF in a diesel engine.